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I'm working on a project for an industrial application where I need to run a monitor with a single cable carrying both data and power. 7" LCD with a few Watts backlight, 1024x600@30fps or so, nothing crazy. Power over Ethernet (PoE) could be a good choice, but for software architecture reasons it would be most convenient to use DisplayPort or HDMI. However, those formats don't carry power and don't come with rugged connectors. Elsewhere in the system we are using PoE over a Cat6 cable with M12 X-coded connectors due to the industrial environment. We get to design the monitor side too so we have flexibility.

It occurs to me that since DP and HDMI are also just four pairs of differential data just like Ethernet, I should be able to simply replicate the 48V PoE++ (using all 4 pairs for power) scheme that I am using elsewhere, but put the DP/HDMI lines on it instead of Ethernet. I'd use standard ethernet transformers with center tap for the PoE DC power. I have read that HDMI over Cat6 is something that can be done, but I can't find anything about integrating PoE-style power transmission over the same wires. To be clear I'm talking about directly putting the DP/HDMI lines on the Cat6 cable, not transmitting video data as Ethernet frames.

Is this a crazy idea? Is there something I'm not understanding? Is there a better single-cable solution for a monitor?

Thanks.

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  • \$\begingroup\$ I doubt the standard DP/HDMI line drivers/receivers will tolerate long distance wiring very well, so you might need to get creative there. \$\endgroup\$
    – brhans
    Jan 18 at 21:29
  • \$\begingroup\$ Thanks for the response. The wiring distances aren't too bad - probably 3-5 meters. I'll look into re-drivers. \$\endgroup\$ Jan 18 at 21:31
  • \$\begingroup\$ Displayport over Ethernet is probably not any industry standard, you also don't know how many conductors they are using. I would say no, run two cables, the best way to find out is to try, or pry open the displayport box and see which conductors it's using. Also I'll reopen the question if this is about designing a box, if it's about using a displayport box it's off topic. \$\endgroup\$
    – Voltage Spike
    Jan 18 at 21:50
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    \$\begingroup\$ Have you considered USB C? Has video alt mode, and power, and totally standard. \$\endgroup\$
    – Ben Voigt
    Jan 18 at 22:36
  • \$\begingroup\$ A) for that resolution, you'll probably be looking at around (or just under) 230 MHz across the four pairs... which seems doable. B) I don't think DisplayPort or HDMI will appreciate the transformers being inline, at all (try it)... and without them you have no way to deal with the "Phantom Power" scheme required by PoE. C) USB-C as mentioned (though it's a big topic to get into), and there are chipsets that support video over reduced pairs (even one pair), depending on your desired bandwidth - I suspect that would be my direction. \$\endgroup\$
    – Attie
    Jan 18 at 23:47

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"since DP and HDMI are also just four pairs of differential data just like Ethernet" is a poor assumption, they really aren't.

DP and HDMI have multiple other important wires too for handshaking, and they cannot pass through CAT cable as it is wrong type of cable (except with some buffer chips) but you would need multiple CAT cables.

Unless you convert the HDMI or DP video stream to pass CAT cables, like you could use HDBaseT or some other modules that convert video to IP streams like some KVM extenders. Both these support PoE as they are intended for transferring video and power through single CAT cable.

Passing HDMI or DP video through PoE transformers would be difficult, even LVDS might suit better for that.

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  • \$\begingroup\$ Thanks for your answer. I know about the handshaking pins but I figured since I control both sides maybe I could put them in a fixed configuration. What do you mean that it's the wrong type of cable for DP/HDMI? It's small gauge shielded twisted pairs, right? I have also read about HDBaseT but that seems like a lot of complexity and overhead. So I guess either a locking HDMI + second power cable, or an M12 with enough pins and twisted pairs for the whole DP/HDMI interface. \$\endgroup\$ Jan 19 at 16:18
  • \$\begingroup\$ Don't some of the displayport extenders just do frame conversion digitally and then send it over ethernet? \$\endgroup\$
    – Voltage Spike
    Jan 19 at 16:50
  • \$\begingroup\$ Yeah, and my system could technically do that on our processor, but it's not great for the software architecture. I'm looking for a way to use the native DP/HDMI interface if possible, perhaps with a re-driver. \$\endgroup\$ Jan 19 at 18:22
  • \$\begingroup\$ @MitchamTuell Sure they are both small gauge shielded twisted pairs - but that's where it stops, they are intended for completely different signals and interfaces. CAT6 requires bandwidth of 250 MHz at 100 meters. DisplayPort requires 1.6 GBps at slowest link speed, which means the bandwidth required could be many multiples of 1.6GHz. It does not even go further than a few meters without active cables or optical cables. HDBaseT is a technology to convert HDMI into something that can go over CAT6 cabling transparently. Basically same thing why GB Ethernet needs CAT6 instead of CAT3 for phones. \$\endgroup\$
    – Justme
    Jan 19 at 18:33
  • \$\begingroup\$ @Justme thanks for the response. Is it possible to buy a chip that can do the HDBaseT conversion with dedicated hardware, or would I have to implement it on my processor? \$\endgroup\$ Jan 19 at 18:40

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